22 Dec 2021
22 Dec 2021
Status: a revised version of this preprint is currently under review for the journal ESurf.

The effect of debris-flow sediment grain size distribution on fan forming processes

Haruka Tsunetaka1, Norifumi Hotta2, Yuichi Sakai3, and Thad Wasklewicz4 Haruka Tsunetaka et al.
  • 1Forestry and Forest Products Research Institute, Ibaraki, Japan
  • 2Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan
  • 3Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, Japan
  • 4Stantec Inc, Environmental Services, Geohazards and Geomorphology Group, Fort Collins, CO, USA

Abstract. Knowledge of the processes driving debris-flow fan evolution are critical in the support of efforts to mitigate related hazards, reduce risk to populations and infrastructure, and reconstruct the history of sediment dynamics in mountainous areas. Research on debris-flow fan development has focused on topographic controls, debris-flow volume and rheology, and the sequence of occurrence of debris flows. While these items have explained a great deal about fan formation and specifically avulsion and runout mechanisms, there is a need to further investigate other properties as they relate to debris-flow fan formative process. Here, we examined the role of debris-flow grain-size distribution on fan formation. Flume experiments were employed to examine the morphology of debris-flow fans that resulted from flows with mono- or multi-granular sediment composition with the same average grain size. All other flow characteristics were held constant. The mono-granular flows formed a symmetric-like fan morphology because there was little avulsion during the formation process. The multi-granular flows produced fans with an asymmetric morphology. Avulsions occurred on both lateral extents of the fan during the early stages of fan development and caused the runout direction to shift produce the fan asymmetry. Grain-size distribution was closely related to spatial diversity in fan morphology and stratigraphy.

Haruka Tsunetaka et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2021-69', Anonymous Referee #1, 16 Jan 2022
  • RC2: 'Comment on esurf-2021-69', Anonymous Referee #2, 20 Jan 2022
  • AC1: 'Comment on esurf-2021-69', Haruka Tsunetaka, 02 Mar 2022

Haruka Tsunetaka et al.

Haruka Tsunetaka et al.


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Short summary
Two simulations are performed using mono-granular and multi-granular flows with the same average grain size, while all the other conditions were kept the same. The mono-granular flow formed a symmetric-like fan morphology as there was less avulsion during formative processes. The multi-granular flows resulted in bilaterally widened avulsions during the early stages of the inundation, and the runout directions shifted as the topography evolved, and resulted in asymmetric fan morphology.